Distribution board
A distribution board is a component of an electricity supply system that divides an electrical power feed into subsidiary circuits while providing a protective fuse or circuit breaker for each circuit in a common enclosure. Normally, a main switch, and in recent boards, one or more residual-current devices or residual current breakers with overcurrent protection are also incorporated.
In the United Kingdom, a distribution board designed for domestic installations is known as a consumer unit.
North America
North American distribution boards are generally housed in sheet metal enclosures, with the circuit breakers positioned in two columns operable from the front. Some panelboards are provided with a door covering the breaker switch handles, but all are constructed with a dead front; that is to say the front of the enclosure prevents the operator of the circuit breakers from contacting live electrical parts within. Busbars carry the current from incoming line conductors to the breakers, which are secured to the bus with either a bolt-on connection or a plug-in connection using a retaining clip. Panelboards are more common in commercial and industrial applications and employ bolt-on breakers. Residential and light commercial panels are generally referred to as load centers and employ plug-in breakers. The neutral conductors are secured to a neutral bus using screw terminals. The branch circuit bonding conductors are secured to a terminal block attached directly to the panelboard enclosure, which is itself grounded.During servicing of the distribution board, when the cover has been removed and the cables are visible, American panelboards commonly have some live parts exposed. In Canadian service entrance panelboards the main switch or circuit breaker is located in a service box, a section of the enclosure separated from the rest of the panelboard, so that when the main switch or breaker is switched off no live parts are exposed when servicing the branch circuits.
Breaker arrangement
Breakers are usually arranged in two columns. In a U.S.-style board, breaker positions are numbered left-to-right, along each row from top to bottom. This numbering system is universal with numerous competitive manufacturers of breaker panels.Each row is fed from a different line, to allow 2- or 3-pole common-trip breakers to have one pole on each phase. In North America, it is common to wire large permanently installed equipment line-to-line. This takes two slots in the panel and gives a voltage of 240 V for split-phase electric power, or 208 V for three-phase power.
Interior
The photograph to the right shows the interior of a residential service panelboard manufactured by General Electric. The three service conductors—two 'hot' lines and one neutral—can be seen coming in at the top. The neutral wire is connected to the neutral busbar to the left with all the white wires, and the two hot wires are attached to the main breaker. Below the main breaker are the two bus bars carrying the current between the main breaker and the two columns of branch circuit breakers, with each respective circuit's red and black hot wires leading off. Three wires can be seen exiting the left side of the enclosure running directly to a NEMA 5-15 electrical receptacle with a power cord plugged into it. The incoming bare, stranded ground wire can be seen near the bottom of the neutral bus bar.The photograph on the left shows a dual panel configuration: a main panel on the right and a subpanel on the left. The subpanel is fed by two large hot wires and a neutral wire running through the angled conduit near the top of the panels. This configuration appears to display two violations of the current U.S. National Electrical Code: the main panel does not have a grounding conductor and the subpanel neutral bar is bonded to the ground bar.
Fuse boxes
A common design of fuse box that was featured in homes built from 1940 through 1965 was the 60-amp fuse box that included four plug fuses for branch circuits and one or more fuse blocks containing cartridge fuses for purposes such as major appliance circuits. After 1965, the more substantial 100 A panel with three-wire service became common; a fuse box could have fuse blocks for the main shut-off and an electric range circuit plus a number of plug fuses for individual circuits.United Kingdom
This picture shows the interior of a typical distribution panel in the United Kingdom. The three incoming phase wires connect to the busbars via a main switch in the centre of the panel. On each side of the panel are two busbars, for neutral and earth. The incoming neutral connects to the lower busbar on the right side of the panel, which is in turn connected to the neutral busbar at the top left. The incoming earth wire connects to the lower busbar on the left side of the panel, which is in turn connected to the earth busbar at the top right. The cover has been removed from the lower-right neutral bar; the neutral bar on the left side has its cover in place.Down the left side of the phase busbars are two two-pole RCBOs and two single-pole breakers, one unused. The two-pole RCBOs in the picture are not connected across two phases, but have supply-side neutral connections exiting behind the phase busbars. Down the right side of the busbars are a single-pole breaker, a two-pole RCBO and a three-pole breaker.
Larger commercial, public, and industrial installations generally use three-phase supplies, with distribution boards which have twin vertical rows of breakers. Larger installations will often use subsidiary distribution boards.
In both cases, modern boards handling supplies up to around 100 A or 200 A use circuit breakers and RCDs on DIN rail mountings. The main distribution board in an installation will also normally provide a main switch which switches the phase and neutral lines for the whole supply.
For each phase, power is fed along a busbar. In split-phase panels, separate busbars are fed directly from the incomer, which allows RCDs to be used to protect groups of circuits. Alternatively RCBOs may be used to provide both overcurrent and residual-current protection to single circuits.
Other devices, such as transformers and contactors may also be used.
New British distribution boards generally have the live parts enclosed to IP2X, even when the cover has been removed for servicing.
Consumer units
In the United Kingdom, BS 7671 defines a consumer unit as "A particular type of distribution board comprising a type tested coordinated assembly for the control and distribution of electrical energy, principally in domestic premises..." These installations usually have single-phase supplies at 230 V ; historically, they were known as fuse boxes, as older consumer units used fuses until the advent of mini-circuit breakers.A normal new domestic CU used as a main panel might have from 6 to 24 ways for devices, and will be split into two or more sections. Secondary CUs used for outbuildings usually have 1 to 4 ways plus an RCD.
Recent CUs would not normally have RCD protected sections for anything other than socket outlets, though some older CUs featured RCD incomers. Before 1990, RCDs were not standard in CUs.
Fuse boxes normally use cartridge or rewirable fuses with no other protective device, and basic 4-ways boxes are very common. Some older boxes are made of brown-black bakelite, sometimes with a wooden base. Although their design is historic, these were standard equipment for new installs as recently as the 1980s, so they are very common. Fuseholders in these boxes may not provide protection from accidental contact with live terminals.
Examples
In the UK, consumer units have evolved from basic main switch and rewireable fuses, that afforded only overload and short circuit protection, into sophisticated control units housing many safety features that can protect against different types of electrical fault. The choice of circuit protective device will depend upon the type of electrical circuit it is protecting and what level of protection needs to be afforded. BS7671:2018 Requirements for Electrical Installations, also referred to as the IET Wiring Regulations, gets regularly updated and its latest edition at the time of writing is amendment 2:2022 released on 28 March 2022.Typical configurations of CU:
- Main switch consumer unit - Consists of a main switch that will disconnect power to all circuits simultaneously which has one busbar linking all protective devices to a common live source, and one neutral conductor or link bar connecting to a common neutral rail. There will be a separate earth rail to allow the main earth conductor to be connected. This example offers the highest degree of circuit separation as all circuits are independent. This particular example amay not be suitable as a standalone solution with only overload and short circuit protection MCBs for each circuit. Additional protection from earth leakage RCBOs faults and arc faults AFDD may be required by BS7671 making this an expensive solution.
- Main Switch and Dual RCD consumer unit - Consists of a main switch that will disconnect power to all circuits simultaneously and two 30mA RCDs RCDs each with its own live busbar each protecting a separate bank of circuits, typically half-and-half but other combinations are available, from earth leakage faults. Offers a cost-effective solution by using a combination of cheaper mcbs and only two, more expensive, RCDs.
- High integrity consumer unit - Consists of a main switch that will disconnect power to all circuits simultaneously and three separate live busbars, one linked directly to the main switch and two others on each main RCD. The live busbar on the main switch allows the use of mcbs only where more sensitive devices such as RCBOs and AFDDs would not be appropriate, or the independent use of RCBOs, and may be limited to only one or two ways. The remainder of the circuits are divided in the same way as a dual RCD CU. This type of consumer unit offers improved circuit separation over a dual RCD CU whilst allowing for more flexibility.
- RCD incomer consumer unit - This is the least convenient solution in terms of circuit separation because the main switch is an RCD. Less common than the other types, it is no longer considered a standalone solution because power to all circuits is lost in the event of an earth fault causing the main switch RCD to activate.
The choice of consumer unit will reflect several factors such as the size and layout of the dwelling, number of floors, outbuildings, the expected loads, and how much protection is required for each circuit.
The box pictured top-right is a "Wylex standard" fitted with rewirable fuses. These boxes can also be fitted with cartridge fuses or miniature circuit breakers. This type of consumer unit was very popular in Britain until 2001 when wiring regulations mandated residual-current device protection for sockets that could "reasonably be expected to" supply outdoor equipment. There were a number of similar designs from other manufacturers but the Wylex ones are by far the most commonly encountered and the only ones for which fuseholders/breakers are still commonly available.
Some manufacturers have added innovative features such as CPN Cudis who have added a LED strip light to their 'Lumo' consumer unit to enhance visibility in dark locations such as under staircases.